Next-generation hearing aids emulate fly's ability to pinpoint sound

There is a fly that can locate a cricket from the sound it makes, despite other noises in
the background. Yet the hearing mechanism that helps the fly do this spans only 1.5 mm, which is
50 times smaller than the wavelength of the cricket's chirp. Now, engineers have found a way
to mimic the fly's super-hearing in a tiny device that does not require a bulky battery.

A paper describing the work is published in the journal Applied Physics Letters.

The engineers, from the Cockrell School of Engineering at the University of Texas at Austin,
say the new device could be used in a new generation of hypersensitive hearing aids that use
intelligent microphones to select only those sounds or conversations that the wearer wants to
hear.

Fly's sophisticated hearing can locate a cricket with remarkable accuracy

Neal Hall, an assistant professor in the Cockrell School's Department of Electrical and
Computer Engineering, and his team of graduate students, drew their inspiration from pioneering
work by Ronald Miles at Binghamton University, NY, and Ronald Hoy at Cornell University, Ithaca,
NY.

The Ormia ochracea fly has a sophisticated sound processing mechanism that determines the direction of a sound within an angle of 2 degrees.

They were the first to describe the technological potential of emulating the super-hearing
mechanism of the yellow-colored parasitoid fly Ormia ochracea, which stalks and locates
male field crickets from their chirps and lays live larvae on and around them.

The fly can locate the cricket with remarkable accuracy because it has a sophisticated sound
processing mechanism that determines the direction of the sound within an angle of 2
degrees.

Using the fly's super-evolved hearing structure as a model, Prof. Hall and colleagues made a
tiny pressure-sensing device out of silicon. With a span of only 2 mm, the device is nearly the
same size as the fly's hearing organ.

Unlike many insects, the reason humans and other mammals can pinpoint the source of a sound is
because we have a much larger distance between our ears. The sound processing mechanism in our
brains uses the time difference in the arrival of the sound at the two ears to locate the
source.

But insects' bodies are generally too small to do this - the sound waves effectively hit both
sides almost at the same time.

That is, except for insects like O. ochracea - it can locate the direction of a
cricket's chirp even though its ears are less than 2 mm apart. Its highly evolved hearing
mechanism can sense the 4 millisecond gap between the sound entering one ear and the other. It
also amplifies this time difference using a "teeter-totter" or "see-saw" mechanism that allows it
to locate the cricket with remarkable accuracy.

Engineers emulated fly's hearing mechanism using a flexible beam

To replicate the fly's hearing mechanism, the team made a flexible beam incorporating piezoelectric materials that
allowed them to use the flexing and rotation of the beam as a way to measure sound pressure and
pressure gradient at the same time.

While other teams have already tried to build hearing devices that emulate the fly's super-hearing, Prof. Neal and colleagues are the first to use piezoelectric materials, which convert
mechanical pressure into electrical signals and allow the device to work with very little
power.

He sees this technology being attractive to people with hearing problems in the future. While
as many as 1 in 10 Americans could benefit from a hearing aid, currently only a fifth of this
number use one, he adds.

He says many believe the main reason for the gap is hearing aid wearers' dissatisfaction with
the devices:

"Turning up the volume to hear someone across from you also amplifies all of the surrounding
background noise - resembling the sound of a cocktail party."

As well as taking hearing aid technology to a new level, the device could also be useful in
military and defense applications. For example, in dark environments where visual cues are
absent.

Funds from the Defense Advanced Research Projects Agency (DARPA) helped finance the study.

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